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Charge your nicad cells in rapid time with this ... By DARREN YATES FAST CHARGER FOR NICAD BATTERIES Tired of waiting for the 16 hours it takes to charge your nicad cells? This low-cost project uses a single Philips IC & will charge four “AA” cells in 50 minutes. It runs from a 12V 1A plugpack supply or from a car battery. Nicad batteries are now one of life’s necessary evils. They can make running battery-operated gear much cheaper than using ordinary dry cells but they do have one big disadvantage – when the batteries go flat, it usually takes about 16 hours to re­charge them. Another disadvantage is their lower output voltage compared to standard dry cells (1.2V vs 1.5V). 18  Silicon Chip We can’t do much about the voltage difference between the two types of batteries but we can do something about the time it takes to recharge nicads. The answer is to build this Fast Nicad Charger. It can charge either two or four “AA”, “C” or “D” cells in rapid time – 50 minutes for “AA” 600mAh cells and 100 minutes for “C” and “D” 1.2Ah cells. The circuit is based on a new Philips chip – the TEA1100. This is a dedicated nicad charger IC with inbuilt switching controllers. This switching technique provides much higher efficiency than the more conventional linear techniques. We’ve used the switching controller feature and several other features of the chip to make one of the simplest yet most comprehensive nicad chargers currently available. It provides automatic cutout when the batteries are fully charged, a timer override and two charging modes – fast and trickle. Preventing overcharging Standard nicad chargers use circuitry which applies a con­stant current Voltage sensing & timing The Fast Nicad Charger uses both current and voltage sens­ing to ensure correct charging, as well as an RC clock/timer which shuts down the circuit after a preset time if the sensing circuit fails to detect the full-charge condition. The charging current is sensed simply by using a low-value resistor in series with the battery but the voltage sensing is somewhat more complicated. Instead of checking the battery vol­ tage for an absolute value, the circuit V CHARGE CURRENT Fig.1: typical charging curve for a nicad cell. Note how the voltage falls slightly at the end of the charging cycle. This is detected by the circuit & used to switch the charging current to a low level to keep the battery topped up. BATTERY VOLTAGE to the battery over a preset period of time – usually about 16 hours for ordinary nicads and five hours for the fast-recharge types. The big disadvantage of this technique is that it doesn’t take into account the current charge state of the battery and this can lead to overcharging and possible damage to the battery pack. By contrast, the Fast Nicad Charger does take the current charge state of the battery into consideration and sets its charging current accordingly. This prevents overcharging and greatly increases battery life. Another problem with nicad batteries is the so-called “memory effect”. Often, batteries are placed into a charger with­ out having been completely discharged beforehand. In the short term, this doesn’t cause too much of a problem but problems do occur after repeated charge/discharge cycles. What happens is that the battery develops a memory for the point to which it is continuously discharged and this ends up becoming the end point for future use. In other words, the bat­ tery will only partially discharge before appearing to go “flat”. This can reduce the effective capacity of the battery by more than half in some cases. The only way to prevent this unwanted memory effect from occurring is to deep-cycle the battery. In practical terms, this means discharging the battery to its recommended end-point vol­tage before placing it in the charger. An automatic discharge circuit is not a feature of this project, however. If you want to correctly discharge nicad bat­teries, we recommend that you build either the Nicad Discharger described in the July 1992 issue of SILICON CHIP or the Automatic Nicad Discharger described in the November 1992 issue. TIME looks for a relative change of 1% from the maximum voltage – see Fig.1. Unlike SLA batteries, once nicads reach their full charge capacity, their output voltage drops. Because it is virtually impossible to predict the absolute maximum voltage, Philips has used an alternative method called “-dV sensing”. By looking for a 1% drop in the relative battery voltage, the new TEA1100 can accurately determine when a nicad pack is fully charged. This ensures that the battery is never overcharged, regardless of its initial capacity. The RC clock/timer utilises a counter block within the TEA1100 to set a maximum timeout period. Its job is to automati­cally switch off the charger if the battery voltage hasn’t dropped the required 1% during the set time period, or if the -dV sensing circuit misses the slight drop in output voltage when the cells are fully charged. Essentially, the timing circuit is included as cheap in­surance against the circuit not shutting down, as can occur if the cells are faulty or if the sensing circuit fails to detect the full charge condition. Some cells have only a very shallow voltage drop at the end of their charging cycle and this can sometimes be missed by the sensing circuitry. In most cases though, by the timer the timer operates, the circuit will have already shut down. Circuit diagram Fig.2 shows the complete circuit details of the Fast Nicad Charger. Power is derived from a 12V DC 1A source and applied to the circuit via on/off switch S1 and reverse-polarity protection diode D1. Since the TEA­ 1100 requires a supply of between 5.5V and 11V, ZD1, Q3 and their associated components form an 8.5V regula­tor which feeds pin 12 of IC1. The output from the regulator also drives charging indicator LED 1 via pin 15. The charging current flows to the batteries from D1 via transistor Q2, a TIP32C 3-amp PNP power device. Main Features • • • • Two charging modes – fast and trickle. • Timer override to ensure charger cuts off if cells are faulty or fully charged condition not detected. • • Can be powered from a 12V 1A plugpack supply or from a car battery. Charges two or four cells (600mAh or 1.2Ah capacity) at once. Charges “AA” cells in 50 minutes & “C” & “D” cells in 100 minutes. Automatically cuts off when cells are fully charged & switches to trickle charge mode. Has reverse polarity protection for power supply & is fully protected against short-circuit or open circuit nicad batteries May 1994  19 C Q3 BC337 S1 12V INPUT L1 : 60T,0.5mm DIA ENCU ON ALTRONICS L-5120 TOROID 10 16VW ZD1 9.1V 400mW Q2 TIP32C C E 470 16VW 470  B 3.3k D1 1N4004 E LED1 CHARGE  L1 10k D2 FR104 B 100  Q1 BC337 C 12 2.2k B E 100pF K A 15 S2 IC1 TEA1100 5 4 13 16 3 27k B CE 0.1  5W S3 470 16VW 600mAH 1.2AH .0018 10 680pF E C VIEWED FROM BELOW 4 CELLS 7 1 2.2k B 100k 2 CELLS 2 OR 4 CELL BATTERY Fig.2: the circuit is based on IC1. It samples the cell voltage via its pin 7 input & provides a pulse width modulated (PWM) output at pin 1. This PWM output drives Q1 and this in turn drives power transistor Q2 which switches current pulses through to the cells. 100k .0039 47k FAST NICAD/NIMH BATTERY CHARGER Along with fast-recovery diode D2 and inductor L1, these components form a step-down DC-DC converter which is pulse width modulated (PWM) con­ trolled by IC1. The pulse-width modulated waveform appears at pin 1 of IC1 and is inverted by transistor Q1. This in turn switches power transistor Q2 to control the current fed to the batteries. Voltage monitoring is achieved by applying a proportion of the output voltage to the Voltage Accumulator input (pin 7). This is done by using S2 to select between one of two voltage divider circuits which connect across the battery. The valid input range for pin 7 is between 0.385V and 3.85V. The maximum charging time is set by switch S3 and its two associ- ated timing capacitors: 0.0018µF for 600mAH batteries and 0.0039µF for 1.2AH batteries. The two capacitors determine the frequency of the timing oscillator; the higher the capacitor value, the lower the frequency and the longer the charging time. The .0018µF capacitor sets the timeout period to 50 minutes, while the .0039µF capacitor sets the period to 100 minutes. Charge LED The TEA1100 uses only a single LED to indicate one of two charging states. When the charger is first switched on, the charge LED is on continuously, indicating that the circuit has gone into the main “fastcharge” mode. Once the circuit has decided that the batteries are charged, the LED flashes. This not only indicates “endof-charge” but also the rate at which the current pulses are being fed to the battery to maintain a “trickle” charge. This trickle charge will maintain the batteries in top condition after the main charging cycle has been completed. The charging current is regulated by the IC and the 2.2kΩ resistor between pin 5 and ground. This, along with the 0.1Ω 5W current sensing resistor on pin 16, sets the main charging cur­rent to just on 960mA. The main internal reference current is determined by the 27kΩ resistor connected to pin 10 and is set to approximately 45µA. In order to main maintain loop sta- RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 2 1 1 1 1 2 1 1 1 20  Silicon Chip Value 100kΩ 47kΩ 27kΩ 10kΩ 3.3kΩ 2.2kΩ 470Ω 100Ω 0.1Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown red violet orange brown brown black orange brown orange orange red brown red red red brown yellow violet brown brown brown black brown brown not applicable 5-Band Code (1%) brown black black orange brown yellow violet black red brown red violet black red brown brown black black red brown orange orange black brown brown red red black brown brown yellow violet black black brown brown black black black brown not applicable S3 PARTS LIST S2 5 2 3 4 A 1 6 LED1 K 470uF Q3 .0039 .0018 47k 2.2k 27k 0.1  5W Q2 1 2 LED1 A K 100  470  3 IC1 TEA1100 L1 4 5 6 1 680pF 100pF D2 2.2k 10k 100k ZD1 12V Q1 10uF 100k D1 3.3k S1 470uF OUTPUT Fig.3: install the parts on the PC board as shown on this wiring diagram, making sure that all polarised parts are correctly oriented. L1 consists of 60 turns of 0.5mm-diameter copper wire on a Neosid toroidal core. Fig.4: check your PC board against this full-size artwork before installing any of the parts. bility, an RC network consisting of a 47kΩ resistor and a 680pF capacitor is connected between pin 4 and ground. This ensures that no oscillation or “motor-boating” occurs by reducing the bandwidth of the circuit while still maintaining an adequate level of error voltage feed­back information. Construction All the parts for the Fast Nicad Charger, except for the three switches and LED 1, are installed on a PC board coded 11102941. Fig.3 shows the assembly details. Before installing any of the parts, it’s a good idea to check the board carefully for any shorts or breaks in the tracks by comparing it with the published pattern (Fig.4). If you do find any, use a small artwork knife or a dash of solder to fix the problem as appropriate. Begin the assembly by installing PC stakes at the external wiring points, then install the wire link, the resistors and diodes. Be sure to use the correct diode type number at each location and make sure that they are all correctly oriented. After that, you can install the MKT capacitors, the elec­trolytics and the 0.1Ω 5W resistor. Next, install the three transistors and the IC, again taking care with the polarity. Once these parts are in, a small finned heatsink should be attached to transistor Q2 using a 3mm machine screw and nut. The last component to go on the board is inductor L1. This is wound on 1 PC board, code 11102941, 102 x 56mm 3 SPDT toggle switches 1 plastic case, 137 x 60 x 42mm 1 micro-U heatsink 1 large black crocodile clip 1 large red crocodile clip 1 small black crocodile clip 1 small red crocodile clip 4 PC stakes 1 5mm LED bezel 1 front panel label 1 33mm OD toroidal core 1 2-metre length of 0.5mm diameter enamelled copper wire Semiconductors 1 TEA1100 battery monitor for nicad chargers (IC1) 2 BC337 NPN transistors (Q1,Q3) 1 TIP32C PNP power transistor (Q2) 1 1N4004 silicon diode (D1) 1 FR104 fast-recovery diode (D2) 1 9.1V 400mW zener diode (ZD1) 1 5mm green LED (LED1) Capacitors 1 470µF 16VW electrolytic 1 100µF 16VW electrolytic 1 10µF 16VW electrolytic 1 .0039µF 63VW MKT polyester 1 .0018µF 63VW MKT polyester 1 680pF 63VW MKT polyester 1 100pF 63VW MKT polyester Resistors (0.25W, 1%) 2 100kΩ 2 2.2kΩ 1 47kΩ 1 470Ω 1 27kΩ 1 100Ω 1 10kΩ 1 0.1Ω 5W 1 3.3kΩ Miscellaneous Screws, nuts, washers, hook-up wire. a Neosid toroidal core (Altronics Cat. L-5120) using two metres of 0.5mm diameter enamelled copper wire. Feed about one half of the wire through the middle on the toroid, then wind on about 30 turns, keeping the windings tight and close together. The other half of the wire can then be used to complete the winding. May 1994  21 screws and nuts, with an additional nut under each corner to serve as a spacer. This done, complete the wiring to the front panel items as shown in Fig.3. Make sure that switches S2 and S3 are oriented with respect to the LED exactly as shown (ie, the switch terminals connecting to points 1 & 6 on the PC board must be nearest the LED). You will also have to connect the power supply and output leads. These can be fitted with crocodile clips or terminated in some other suitable manner, depending on your power supply and the terminals on your nicads or their holder. Testing Once everything is in position, connect your multimeter (set to the 2A Plastic cable ties are used to secure the wiring to the two switches & to anchor range) in series with the power supply the large toroidal inductor to the PC board. Take care to ensure that switches and switch on. You should find that S2 & S3 are correctly oriented on the front panel – see text & Fig.3. the quiescent current measures about 5-10mA and that the LED is off. The exact number of turns is not for the two switches and the indicator If this checks out, set S2 and S3 to critical but you should find that you LED. It’s best to use a 3mm drill to match your nicad bat­tery pack and get about 60 turns on in total. begin with and then slowly ream the check that the output voltage is close Finally, trim off the excess lead holes to the correct size with a tapered to the mark – for two cells, it should be lengths, clean the wire ends and sol- reamer. somewhere around 2.4V and for four der the inductor into position on the The power switch (S1) is mounted cells it should be about 4.8V. board. The inductor can be anchored on one end of the case and an addiAssuming that the open-circuit using a plastic cable tie which feeds tional hole is drilled adjacent to this output voltage is correct, connect through a hole in the PC board – see to provide access for the power leads. the nicad pack to the output. You photo. A hole drilled in the opposite end of should find that the current drain the case is used for the battery output is now either about 600mA or 1.2A, Final assembly leads. In addition, you will have to depending on the setting of S3, and The board and its associated com- drill four mounting holes in the base that the LED is lit. ponents are in­stalled in a small zippy of the case for the PC board. Depending on how much charge is The various items of hardware can in the battery and the setting of S3, box measuring 137 x 60 x 42mm. First, attach the adhesive label to the lid of now be mounted in posi­tion and the the LED should stay on for some time the case, then drill out mounting holes PC board secured using 3mm machine (it could be as long as 50 minutes for “AA” cells” or 100 minutes for “C” or “D” cells) and then begin to flash. When this flashing begins, the current should drop CHARGE to about 10mA between flashes and rise sharply each time the LED lights. If the LED fails to light, check that it has been oriented 2 600 correctly. Now you can attack that FAST NICAD drawer full of nicad cells and CHARGER charge them up in quick time! Don’t forget though – if you 1200 4 want maximum performance mAH CELLS from your nicad cells, you should also build a discharger to discharge the battery pack to its correct end-point voltage Fig.5: this full-size artwork can be used as a drilling template for the lid of the case. SC before charging. Drill small pilot holes first, then carefully ream these to size. 22  Silicon Chip